Lithographic printing, i.e., offset printing is a printing method comprising applying a desensitizing solution on a printing plate precursor having thereon an image area comprising an ink-receptive lipophilic layer and a non-image area to thereby form a hydrophilic layer on the non-image area, applying an oily ink to the lipophilic image area, and transferring the ink on the image area to paper.
Of the printing plate precursors, an electrophotographic lithographic plate precursor comprising a support, such as paper, having provided thereon a photosensitive layer comprising a photoconductive powder, such as zinc oxide, dispersed in a binder resin is produced by forming an image by an electrophotographic technique. That is, the photosensitive layer is charged, imagewise exposed to light, and developed with a developing solution containing lipophilic toner particles to form an image area. A desensitizing solution is then applied whereby the desensitizer in the desensitizing solution and the photoconductive powder on the surface form a hydrophilic substance which forms a hydrophilic non-image area. The thus prepared lithographic printing plate comprising a lipophilic area and a hydrophilic area is mounted on a printing machine.
The main components of conventional desensitizing solutions are roughly divided into cyan substances and non-cyan substances.
Cyan substances, which contain a cyanide ion in the molecule thereof as an inorganic complex, include ferrocyanides and ferricyanides. These substances exhibit powerful desensitizing ability and provide printing plates satisfying printing characteristics as a whole. Labile to light or heat, however, the cyan substances easily undergo discoloration or sedimentation or reduce their desensitizing ability with time. Further, the cyan substances themselves are stable and harmless to human bodies but are decomposed under various environmental conditions, such as irradiation of ultraviolet rays or a radiation, to release harmful cyanide ions, which may cause environmental problem. Furthermore, where a plate produced by using a cyan substance-based desensitizing solution is used for printing on neutral paper or printing with quick-drying color inks, such unfavorable phenomena as stains on prints and emulsification of inks tend to occur.
The non-cyan substances, on the other hand, include phytic acid or a salt thereof and, in addition, inorganic salts. Although phytic acid or a salt thereof is excellent in environmental safety and workability, it has weak desensitizing ability so that conditions of printing are difficult to set and the prints are liable to staining. In order to overcome these disadvantages of phytic acid, a combined use with a metal complex of an aminocarboxylic acid (see JP-B-2-39397, the term "JP-B" as used herein means an "examined published Japanese patent application") and a combined use of a hexametaphosphoric acid salt (see JP-B-62-7597) have been suggested. However, satisfactory effects have not been obtained yet.